This invention relates to an improved method and apparatus of production of synthetic multifilamentary yarns having uniform quality from high molecular weight linear polymers, in particular polyesters, according to an improved melt spinning process.
An important area of use of such synthetic multifilamentary yarns is the production of tire cord. A number of high polymers are well suited for this utility, especially polyesters and polyamides; however, in the following description reference will be made particularly to filaments of polyethylene terephthalate.
Since tire cord and the structures formed from it are among the essential construction elements for the safety and useful life of a tire, high quality requirements are naturally placed on such endless filaments. In view of the alternating stretching and compression stresses which tires experience in operation, a necessary precondition for the use of synthetic multifilamentary yarns for tire cord is an adequate fatigue resistance of the filaments. For optimum results, it is critical that the individual filaments be substantially uniform. Accordingly, it is common practice to determine the coefficient of variation of the evenness of the yarn (U %) using an Uster evenness tester as manufactured by the Zellweger Company of Uster, Switzerland, and described in "Handbook of Textile Testing and Quality Control" by E. Groover and D. S. Hamby.
Polyethylene terephthalate has come into strong prominence in the last few years for use in tire cord production. Polyethylene terephthalate unfortunately undergoes a considerable thermal decomposition between the conclusion of the production of the spinning raw material (raw polymer melt) and its subsequent shaping into threads. This thermal decomposition can be reduced if the molten spinning raw material is maintained for as short a time and at as low a temperature as possible. However, the residence time of the spinning melt in the spinning apparatus is prescribed by the dimensions of the apparatus, and the lower limit of the spinning temperature is determined by the highly undesirable condition of melt fracture. Where melt fracture occurs, the spun, unstretched filaments do not have a smooth or even surface, and exhibit fluctuations in diameter which are unacceptable for use in tire cord.
It is evident from this that the spinning requirements are diametrically opposed. On the one hand, low melt temperature is required for low decomposition, and on the other hand, high spinning temperature is required for trouble-free spinning. It has been suggested that this problem may be overcome by supplying the polymer melt for melt spinning at a temperature below the spinning temperature and heating the melt prior to filament formation. Normally, the required increase in the temperature of the melt is accomplished by use of a spinning assembly that includes a spinning filter disposed upstream of the spinneret plate, the pressure drop across said spinning filter being at least about 150 atmospheres.
Unfortunately, polyester yarn made in accordance with known processes is not completely satisfactory. In particular, fused filaments and excessive variation in the evenness of filaments (U %) has been noted when the polymer melt is extruded through the spinneret at a rate of about 50 pounds per hour or greater. Problems in fiber uniformity have been particularly troublesome in so-called double-end melt spinning of synthetic fibers, i.e., using one spin pot to feed both sides of a "split" spinneret. Accordingly, research has been continued in an effort to solve these deficiencies.